Just go to amazon, and look at what people who have bought those books have bought.

Long Range Goals, Beau Dure - a history of MLS.
Distant Corners, David Wangerin - vignettes about individual subjects in US soccer history (e.g. Bill Jeffrey at Penn State, the NASL's Oakland Clippers, etc.)

Colin Jose has some excellent statistical references for the ASL (1921-1933) and the NASL.